Perhaps the most stringent requirement for land navigation is imposed by winter operations in the high arctic. The serious consequences of being lost or failing to find a supply dump in the arctic make reliability and accuracy of navigational systems vital necessities. Because of weather related poor visibility conditions, it may also be necessary to navigate by instrument very close to the destination in order to actually find it. Furthermore, a combination of circumstances conspire to make the arctic a particularly difficult area in which to navigate by conventional means.
The Canadian arctic, from 60.degree. to 85.degree. latitude, consists of about 7,000,000 square kilometers, of which roughly half is land. To the untrained eye of a non-inhabitant, most of this area appears completely barren and featureless, especially during the long winter period where land-water division is obscured. Certainly above the tree line any movement beyond line of sight requires some non-trivial navigation capability. There are generally no rail or road systems and, in most areas, no permanent recognizable landmarks. Accordingly, normal map reading skills are completely inadequate. Frequent extended periods of low visibility also interfere with visual navigation and make celestial navigation unreliable at best. In any case, sextants and sun compasses are not sufficiently accurate.
In many ways, arctic navigation is similar to desert navigation, except that there are several added complications. The area in question is unique in containing the north geomagnetic pole, which makes the use of a hand-held magnetic compass futile over most of this area because the geomagnetic field lines at the magnetic pole are vertical whereas a compass relies on the horizontal component of the field. It is well known that random fluctuations in the direction of the field vector about its mean value will induce heading errors of magnitude inversely proportional to the horizontal field strength.
While various navigational aids are available, there is currently no vehicle navigation system capable of operating under conditions described above. Radio navigation aids generally do not extend coverage through the arctic. The "Transit" satellite system provides some position fixing capability provided that velocity and altitude measurements are continuously available. Since the Transit satellites are in polar orbits, fixes can be obtained more frequently at higher latitudes, with more than one fix per hour expected above 60.degree.. The accuracy of a Transit position fix depends upon the accuracy of the velocity provided, but can be quite adequate, especially with a 2-channel receiver (to remove the ionospheric effect) in stationary mode. When it becomes available, the Global Positioning System (GPS) will provide position and velocity data quite adequate, even with a C/A code receiver. Until it is fully operational however, the gaps in coverage prevent total reliance on GPS.
This still leaves a need to deadreckon (DR) between Transit fixes or through GPS gaps, so that vehicle velocity determination cannot be avoided. The length of the velocity vector, or the speed, can be easily estimated by using a vehicle odometer output; however, vehicle heading presents some difficulty. The first alternative to magnetic direction finding is gyrocompassing which is based on the fact that the horizontal component of the earth's rotation rate is in the north direction. Gyrocompassing becomes more difficult at higher latitudes where the horizontal earth rate decreases quickly as one moves north. Thus, the gyrocompassing accuracy of a given sensor varies as the secant of the latitude. In practice, for each gyro, there is a latitude above which it will not settle at all. For moderately priced units suitable for land vehicles, this is typically given as about 80.degree.. The present location of the geomagnetic pole is such that the land area where magnetic heading is not available (using a magnetometer) probably does not intersect the land area where gyrocompassing is not possible. Neither of these regions is sharply delineated in fact. Rather each heading measurement becomes increasingly less accurate moving towards and into its corresponding region. The point at which each measurement becomes useless depends on the particular sensors chosen, how they are used, and the magnetic conditions at the time. The important point to note is that in the extreme northwest, which is the area of greatest difficulty, the two methods complement each other, with magnetic heading improving as gyrocompass heading deteriorates and vice versa.